Valve seat arrangement
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an exhaust port for a cylinder block of an internal combustion engine. More particularly, the present invention is concerned with a seating mechanism between the exhaust port of the cylinder head of the engine block and a valve. 
     2. Description of the Related Art 
     An example of a prior art cylinder head of the engine block apparatus is shown in FIGS. 6 and 7. In FIG. 6, a cylinder head of the engine block 50 is of an internal combustion engine (not shown). A camshaft 51 is rotatably mounted in the cylinder head 50. A valve mechanism 52 is operatively connected to the camshaft 51. A valve shaft 53 is operated in accordance with the operation of the valve mechanism 52. The valve mechanism 52 includes a valve rocker arm 54, a valve stem 55, a retainer 56 and a valve spring 57. The valve shaft 53 is arranged between the valve mechanism 52 and the cylinder head 50. An exhaust conduit 58 is formed in the cylinder head 50. A valve seat 59 is arranged at the opening portion of the exhaust conduit 58. The valve seat 59 is formed of heat-resisting steel. 
     FIG. 7 shows a partial cross sectional view of a valve seat portion VS of FIG. 6. In FIG. 7, the valve seat 59 is snugly fitted in the cylinder head 50 by a pressing operation. 
     The valve camshaft 51 rotates in accordance with the operation of the internal combustion engine (not shown). Since the camshaft 51 is provided with sections formed into eccentric, cam-type shape, the valve rocker arm 54 converts a rotational motion into a reciprocating motion about the rocker arm shaft 54a. In accordance with the above mentioned operation, the valve stem 55 pushes down the valve shaft 53. When the valve shaft 53 is pushed down, a cylinder (not shown) and the exhaust conduit 58 communicate each other. On the other hand, when the valve shaft 53 is not pushed down the valve shaft 53 is seated to the valve seat 59. 
     The valve seat 59 is made of heat-resisting steel and the cylinder head 50 is made of cast aluminum. Based on the difference of the coefficient of heat expansion between the material of the valve seat 59 and the cylinder head 50, a gap may be generated between the cylinder head 50 and the valve seat 59. When the gap occurs, the valve seat 59 may separate from the cylinder head. 
     SUMMARY OF THE INVENTION 
     It is a primary object of this invention to provide a ceramic valve sealing mechanism in an internal combustion engine. 
     It is another object of this invention to provide an assembling structure for the ceramic valve sealing mechanism. 
     These objects and advantages are achieved by providing a cylinder head, a combustion chamber in the cylinder head, a valve mechanism in the cylinder head, a exhaust conduit and a valve shaft arranged between the valve mechanism and the cylinder block. The valve shaft operates to provide communication between the exhaust conduit and the combustion chamber. The cylinder head has a valve seating portion at an opening of the exhaust conduit and a valve seat which is made of ceramics is provided at the valve seating portion of the cylinder head. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents a partial cross-sectional view of a cylinder block for an internal combustion engine according to the present invention. 
     FIG. 2 represents an enlarged cross-sectional view of a valve seat portion of FIG. 1. 
     FIG. 3 represents a cross-sectional view of a first embodiment of a valve seat. 
     FIG. 4 represents a cross-sectional view of a first embodiment of a valve seat holder. 
     FIG. 5 represents an enlarged cross-sectional view of a second embodiment. 
     FIG. 6 represents a partial cross-sectional view of a prior art cylinder block portion of an internal combustion engine. 
     FIG. 7 represents an enlarged cross-sectional view of a valve seat portion of FIG. 6. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 through 4 represent a first embodiment of the present invention. FIG. 1 represents a cylinder head of an engine portion of an internal combustion engine. Referring to the FIG. 1, a cylinder head 10 is shown as a portion of an internal combustion engine (not shown). A camshaft 11 is rotatably mounted in to the cylinder head 10. A valve mechanism 12 is operatively connected to the camshaft 
     A valve shaft 13 is operated in accordance with the operation of the valve mechanism 12. The valve mechanism 12 includes a valve rocker arm 14, a valve stem 15, a retainer 16 and a valve spring 17. The valve shaft 13 is arranged between the valve mechanism 12 and the cylinder head 10. An exhaust conduit 18 is formed in the cylinder head 10. A valve seat 19 and a valve seat holder 20 are arranged at the opening portion of the exhaust conduit 18. In this embodiment, the valve seat 19 is made of ceramic material. 
     The camshaft 11 rotates in accordance with the operation of the internal combustion engine (not shown). The camshaft 11 is provided with a plurality of sections having eccentric, cam-type shapes. The valve rocker arm 14 converts rotational motion into reciprocating motion about rocker arm shaft 14a. In accordance with this operation, the valve stem 15 operates the valve shaft 13. When the valve shaft 13 is pushed down, a combustion chamber 10a and the exhaust conduit 18 communicate each other. On the other hand, when the valve shaft 13 is not pushed down, the valve shaft 13 seats against the valve seat 19. 
     FIG. 2 shows a partial cross-sectional view in an enlarged scale of a valve seat portion 21 of FIG. 1. The valve seat 19 and the valve seat holder 20 are pressed into the opening portion of the exhaust conduit 18, so the valve seat 19 and the valve seat holder 20 are snugly fitted into or tightly connected to the cylinder head 10. In this first embodiment, the valve seat 19 is made of ceramic material, the cylinder head 10 is made of cast aluminum and the valve seat holder 20 is steel. The valve seat 19 is formed into a ring-shape. A groove 19a is formed on an outer side surface 19b of the valve seat 19. The groove 19a is formed along the entire circumference of the side surface 19b. 
     FIG. 3 shows a cross-sectional view of the valve seat 19. A taper 19c is formed at a lower portion of the opening portion of the valve seat 19. The angle of the taper 19c is formed dependent on the shape of the valve shaft 13. An opening 19d is formed at the center portion of the valve seat 19. The diameter of the opening 19d corresponds to the diameter of the exhaust conduit 18. 
     The valve seat holder 20 is connected to the outer periphery of the valve seat 19. The valve seat holder 20 is divided into two portions, a first valve seat holder 20a and a second valve holder 20b each of which is of a semi-circular shape (FIG. 4). Continuous projections 20c and 20d form an inner surface of the valve seat holder. At the outer periphery of the valve seat holders 20a and 20b, flanges 20e and 20f are formed. The flanges 20e and 20f connect to the opening of the cylinder head 10 leading to the exhaust conduit 18. The first valve seat holder 20a and the second valve seat holder 20b connect with the outer periphery of the valve seat 19. The continuous projections 20c and 20d connect with the groove 19a of the valve seat 19. The valve seat 19 and the valve seat holder 20 constitute a valve seat portion. The valve seat portion is press fitted into the cylinder head 10 under a predetermined pressure so that the valve seat portion is s fitted into or tightly fixed to the cylinder head 10. 
     FIG. 5 represents a second embodiment of the present invention. A projection 19e is provided on the outer surface of the valve seat 19. A notch 20g is formed on the inner surface of the valve seat holder 20. The notch and projection cooperate to maintain the valve seat in position in the cylinder head. The valve seat holder is divided into two sections as in previous embodiments. Also, the valve seat holder is press fit into the cylinder head so that the valve seat is secured therein. 
     The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing application. The invention which is intended to be protected herein should not, however, be construed as limited to the particular forms disclosed, as these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the present invention. Accordingly, the foregoing detailed description should be considered exemplary in nature and not limited to the scope and spirit of the invention as set forth in the appended claims.